mine is all steel but since I built it myself I will post it.
Before I had steel on the sides and bottom

Before I had the sides on


If your its just for light hauling I see no problem building a wooden flatbed, but depending on how long you want it to last you are probably better off taking the time and spending the money to build a steel frame and use wood for the decking. Thats how the bed on my truck was but after years of neglect most of the decking was rotted out and 95% of the steel was so rusted that I had to cut it off. I guess to answer your question it may be more of an issue with time than how hard the project is. I would guess from start to finish on my bed I have probably close to 100 hours in it, but you could do a wood bed for a F150 in half that time I bet.

Post some pictures if you build something!